Radio mini-halos and AGN heating in cool core clusters of galaxies

The brightest cluster galaxy (BCG) in the majority of relaxed, cool core galaxy clusters is radio loud, showing non-thermal radio jets and lobes ejected by the central active galactic nucleus (AGN). Such relativistic plasma has been unambiguously shown to interact with the surrounding thermal intra-cluster medium (ICM) thanks to spectacular images where the lobe radio emission is observed to fill the cavities in the X-ray-emitting gas. This `radio-mode AGN feedback' phenomenon, which is thought to quench cooling flows, is widespread and is critical to understand the physics of the inner regions of galaxy clusters and the properties of the central BCG. At the same time, mechanically-powerful AGN are likely to drive turbulence in the central ICM which may contribute to gas heating and also play a role for the origin of non-thermal emission on cluster-scales. Diffuse non-thermal emission has been observed in a number of cool core clusters in the form of a radio mini-halo surrounding the radio-loud BCG on scales comparable to that of the cooling region. This contribution outlines the main points covered by the talk on these topics. In particular, after summarizing the cooling flow regulation by AGN heating and the non-thermal emission from cool core clusters, we present a recent study of the largest collection of known mini-halo clusters (~ 20 objects) which investigated the scenario of a common origin of radio mini-halos and gas heating. We further discuss the prospects offered by future radio surveys with the Square Kilometre Array (SKA) for building large (>> 100 objects), unbiased mini-halo samples while probing at the same time the presence of radio-AGN feedback in the host clusters.Comment: 8 pages, 3 figures. Conference proceeding of "The many facets of extragalactic radio surveys: towards new scientific challenges", 20-23 October 2015, Bologna, Ital

Multifrequency VLA radio observations of the X-ray cavity cluster of galaxies RBS797: evidence of differently oriented jets

We report on the peculiar activity of the radio source located at the center of the cooling flow cluster RBS797 (z=0.35), the first distant cluster in which two pronounced X-ray cavities have been discovered. New multifrequency (1.4, 4.8, and 8.4 GHz) observations obtained with the Very Large Array clearly reveal the presence of radio emission on three different scales showing orientation in different directions, all of which indicates that RBS797 represents a very peculiar case. The lowest resolution images show large-scale radio emission characterized by amorphous morphology and a steep spectrum, extended on a scale of hundreds of kpc. On a scale of tens of kpc, there is evidence of 1.4 GHz radio emission elongated in the northeast-southwest direction exactly towards the holes detected in X-rays. The highest resolution image shows the details of the innermost 4.8 GHz radio jets on a kpc scale; they are remarkably oriented in a direction that is perpendicular to that of the extended structure detected at a lower resolution. We therefore find evidence of a strong interaction between the central radio source and the intra-cluster medium in RBS797. We suggest a scenario in which the 1.4 GHz emission filling the X-ray cavities consists of buoyant bubbles of radio emitting plasma that are created by twin jets in the past and whose expansion has displaced the thermal gas that was formerly in the X-ray holes, whereas the two jets visible at 4.8 GHz are related to the present nuclear activity that has restarted at a different position angle from the original outburst that created the outer radio lobes. The total radio luminosity is ~ 10^42 erg/s, corresponding to a factor of a few thousand times less than the estimated cooling luminosity.Comment: 9 pages, 4 figures, accepted for publication in Astronomy & Astrophysics; replaced with revised version corrected for language editin

On the connection between radio mini-halos and gas heating in cool core clusters

In this work, we present a study of the central regions of cool-core clusters hosting radio mini-halos, which are di use synchrotron sources extended on cluster-scales surrounding the radio-loud brightest galaxy. We aim to investigate the interplay between the thermal and non-thermal components in the intracluster medium in order to get more insights into these radio sources, whose nature is still unclear. It has recently been proposed that turbulence plays a role for heating the gas in cool cores. A correlation between the radio luminosity of mini-halos, $\nu P_{\nu}$, and the cooling flow power, $P_{\rm CF}$, is expected in the case that this turbulence also plays a role for the acceleration of the relativistic particles. We carried out a homogeneous re-analysis of X-ray Chandra data of the largest sample of cool-core clusters hosting radio mini-halos currently available ($\sim20$ objects), finding a quasi-linear correlation, $\nu P_{\nu} \propto P_{\rm CF}^{0.8}$. We show that the scenario of a common origin of radio mini-halos and gas heating in cool-core clusters is energetically viable, provided that mini-halos trace regions where the magnetic field strength is $B \gg 0.5 \mu G$.Comment: 4 pages, 2 figures. Conference proceeding of "The many facets of extragalactic radio surveys: towards new scientific challenges", 20-23 October 2015, Bologna, Ital

Radio-continuum surveys with SKA and LOFAR: a first look at the perspectives for radio mini-halos

Diffuse synchrotron radio emission has been observed in a number of cool-core clusters on scales comparable to that of the cooling region. These radio sources are called `mini-halos'. In order to understand their origin, which is still unclear, joint radio and X-ray statistical studies of large cluster samples are necessary to investigate the radio mini-halo properties and their connection with the cluster thermodynamics. We here extend our previous explorative study and investigate the perspectives offered by surveys in the radio continuum with LOFAR and SKA, in particular examining the effect of the intra-cluster magnetic field in the mini-halo region for the first time. By considering the minimum flux detectable in radio surveys and exploiting the $P_{radio}-L_X$ correlation observed for known mini-halos, we estimate the detection limits achievable by future radio observational follow-up of X-ray cluster samples, such as HIFLUGCS and eROSITA. This allows us to estimate the maximum number of radio mini-halos that can potentially be discovered in future surveys as a function of redshift and magnetic field strength. We show that future radio surveys with LOFAR and SKA1 (at 140 MHz and 1.4 GHz) have the potential to discover ~1,000-10,000 radio mini-halo candidates up to redshift z=1. We further note that future SKA1 radio surveys at redshift z>0.6 will allow us to distinguish between different magnetic fields in the mini-halo region, because higher magnetic fields are expected to produce more powerful mini-halos, thus implying a larger number of mini-halo detected at high redshift. For example, the non-detection with SKA1 of mini-halos at z>0.6 will suggest a low magnetic field (B < few $\mu$G). The synergy of these radio surveys with future X-ray observations and theoretical studies is essential in establishing the radio mini-halo physical nature. [abridged]Comment: Accepted for publication in A&A; 9 pages, 9 figures. Revised to match the corrected version after language editin

New JVLA observations at 3 GHz and 5.5 GHz of the `Kite' radio source in Abell 2626

We report on new JVLA observations performed at 3 GHz and 5.5 GHz of Abell 2626. The cluster has been the object of several studies in the recent years due to its peculiar radio emission, which shows a complex system of symmetric radio arcs characterized by a steep spectrum. The origin of these radio sources is still unclear. Due to their mirror symmetry toward the center, it has been proposed that they may be created by pairs of precessing jets powered by the inner AGN. The new JVLA observations were requested with the specific aim of detecting extended emission on frequencies higher than 1.4 GHz, in order to constrain the jet-precession model by analyzing the spectral index and radiative age patterns alongs the arcs. We performed a standard data reduction of the JVLA datasets with the software CASA. By combining the new 3 GHz data with the archival 1.4 GHz VLA dataset we produced a spectral index maps of the extended emission, and then we estimated the radiative age of the arcs by assuming that the plasma was accelerated in moving hot-spots tracing the arcs. Thanks to the high sensitivity of the JVLA, we achieve the detection of the arcs at 3 GHz and extended emission at 5.5 GHz. We measure a mean spectral index <-2.5 for the arcs up to 3 GHz. No clear spectral index, or radiative age, trend is detected across the arcs which may challenge the interpretation based on precession or put strong constraints on the jet-precession period. In particular, by analyzing the radiative age distribution along the arcs, we were able to provide for the first time a time-scale < 26 Myr of the jet-precession period.Comment: 8 pages, 5 figures. Accepted for publication in A&